There's an idea - common, but not popular in scientific circles - that all life on Earth was seeded from comets, asteroids or meteors which struck the planet and contained the building blocks necessary to kickstart the evolutionary process.

It's called "panspermia" and it caused an all-in boffin barney some 15 years ago when several scientists backed claims there was evidence of life in a Martian meteorite found in the Allan Hills in Antarctica.

It's been proven that life can survive in space, just as it's been proven that the interior of the Mars meteorite never rose above 50C during its journey to Earth.

Unfortunately for alien-spotters, those cells can also be found on Earth, so the argument's never been settled.